In this paper, we propose a Maritime Underwater Navigation System (MUNS) for Unmanned Underwater Vehicles (UUVs). The proposed MUNS has a hierarchical architecture comprising of a Wide Area Planner (WAP) and a Local Area Planner (LAP). MUNS is intended to be a plug-and-play module to provide obstacle avoidance and path planning capabilities to existing UUV platforms thereby expanding operational capabilities in cluttered and littoral environments. MUNS is designed to be flexible and to support a variety of tasks including point-to-point moves, object inspection, contour tracking, and region scanning/searching. The performance of MUNS has been validated through extensive simulation studies.